The invention relates to a method for determining the coefficient of friction of disk brakes, in particular of disk brakes of a self-energizing design.
In fluidically activated drum brakes and disk brakes which are customary in series production, the braking effect of the individual wheel brakes is approximated, for example, computationally assuming what is referred to as a “calculation coefficient of friction” exclusively by measuring the brake application forces of the respective brake. The brake application forces are determined indirectly by sensing the activation pressure on the hydraulic or pneumatic brake cylinder.
Deviations of the average coefficient of friction of all the brakes of a brake system can be determined by comparing the predefined deceleration and the achieved deceleration. The achieved deceleration is determined here by evaluating the rotational speed behavior of the wheels using the rotational speed sensors which are present in the brake system, and said achieved deceleration is calculated in a computing unit, for example an ABS or EBS system.
However, it is not possible to make definitive statements about the actual coefficient of friction behavior of individual brakes of the vehicle. Different deviations of the coefficient of friction behavior of the brakes of one axle, between the brakes of the individual vehicle axles or between the brakes of a towed vehicle with respect to a towing vehicle can have very adverse effects on the coordination of the brakes.
This fact is of particular importance in brakes which use self-energizing effects to reduce the activation energy because in such brakes the problem occurs that the differences in coefficient of friction are also amplified so that deviations of the actual coefficient of friction from the calculation coefficient of friction form an excessive proportion of the braking effect which is achieved by the respective brake.
Methods are already known in which, taking into account the cause and effect relationshipFR=μ·FN
  μ  =            F      R              F      N      
FN:=normal force (tensioning force) of the brake
FR:=frictional force (circumferential force on the brake disk)
μ:=coefficient of friction of brake lining/brake disk,
the value of μ can be determined through direct or indirect measurement of FR and FN.
The direct determination is carried out, for example, in the measurement of, for example, component tension by means of sensors which are used specifically for that purpose.
The indirect measurement is carried out, for example, by means of an evaluation of available data, for example using data relating to the current consumption and to the position of the drive motor, from which data the variables to be determined can be obtained on the basis of a given cause and effect relationship using, to a certain extent, complex computing operations. Such a solution is proposed by German Patent Publication No. DE 101 51 950 A1.
The disadvantage of direct determination is the cost of additional sensors and the increased susceptibility of the system to errors.
Indirect determination is based on very complex cause and effect relationships which are severely susceptible to errors owing to a large number of influencing variables and require very costly computing operations.
In the abovementioned document German Patent Publication No. DE 101 51 950 A1, there is also a description of a possibility of replacing the determination of the frictional force by a cause and effect relationship between the frictional force and actuator force in conjunction with the coefficient of friction and the employed wedge angle of the self-energizing mechanism. This relationship can be used in self-energizing mechanisms in which the actuator force acts parallel to the frictional surface, that is to say does not have a direct effect on the tensioning force of the brake.
The coefficient of friction can then be determined from the relationship
      μ    =                  tan        ⁢                                  ⁢        α            -                        F          A                          F          N                      ;            α      ⁢              :              =          wedge      ⁢                          ⁢      angle        ;            F      A        =          actuator      ⁢                          ⁢              force        .            
The actuator force and the normal force can be determined from the current consumption (actuator force) and the position of the drive motor (normal force) without an additional sensor system. However, close consideration shows that this determination is subject to relatively large errors.
In addition to fabrication-related tolerances of the motor, operating influences of the motor, in particular its temperature and a series of mechanical efficiency values of the step down gear mechanism and adjustment mechanism, also play a part in the relationship between the motor current and actuator force.
The relationship between the actuator position and the normal force (tensioning force) of the brake is determined by the overall elasticity of the spread-apart brake caliper. In addition to the tolerances of the brake caliper and of the brake application system of the brake, this relationship is also influenced by the deformation behavior of the brake linings. In addition to the wear state of the brake linings, their compressibility is influenced to a great degree by the temperature in the lining material. The temperature dependence is nonlinear and the temperature distribution in the lining material is nonhomogenous as a function of time owing to the relatively low conductivity of heat of the customary organically bound brake linings.
Idle travel, which the actuator has to overcome during the brake application movement, and the possible variations in the value of this idle travel are very disruptive to the determination of the tensioning force (normal force) of the brake by means of the actuator position. Such idle travel is, in particular, the air clearance of the brake and the play which occurs in various ways in the transmission path from the drive motor to the actual activation element of the brake.
Since the actuator position, i.e. the position of the activation element, is brought about by sensing the angular position of the drive motor taking into account the overall transmission ratio between the motor and actuator, an incorrect determination of the position also results in an error. With the incremental position signal transmitters which are generally used, particularly high-dynamic adjustment processes may lead to individual angular steps which are carried out by the drive motor being sensed incorrectly. Such errors can add up when there are repeated adjustment processes and therefore lead to unacceptable deviations of the actuator position.
Since in the relationship on which the determination of μ is based
  μ  =            tan      ⁢                          ⁢      α        -                  F        A                    F        N            both determination variables FA and FN can be determined only with very large errors, the determination of the coefficient of friction according to the described method is inadequate.
Against this background, the object of the invention is to provide a method for determining the coefficient of friction of the brake lining of disk brakes, in particular of self-energizing disk brakes, which method preferably permits reliable and sufficient information to be obtained about the coefficient of friction of the brake lining without an additional sensor system and by using unambiguous and simple cause and effect relationships.
According to the invention, the coefficient of friction is determined using the following variables:                motor current for brake application direction and retraction direction Iapplic,Iret         no load current I0         wedge angle α        transmission constant K        tensioning force FN.        
This way of determining the coefficient of friction which is simple and yet sufficiently accurate for practice is possible in particular under defined conditions during test brake activations which are initiated specifically for this purpose.
The combination with air clearance adjustment processes provides the advantage that in this context the brake linings are in any case applied briefly with little force to the brake disk. In this situation, all the disruptive idle travel, including the air clearance, is eliminated from the tensioning force transmission path. Taking into account the wear state, which has been determined by the electronically controlled adjustment system, and the temperature state which is known for the time of the test brake activation, the actuator is moved into an actuator position which, under the known peripheral conditions, results in a predefined value of the tensioning force which remains the same for all the test brake activations.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings.